This invention relates to a bias device for a copying machine, which operates to apply a bias voltage to an electrode which confronts the photo-sensitive member.
FIG. 1 is an explanatory diagram showing the arrangement of a copying machine to which the technical concept of this invention is applied. Provided around a photosensitive member 1 rotating in the direction of the arrow A are a discharging charger 2, a charging charger 3, an electrode 5 for applying a voltage produced by a bias device 4, and a transferring charger 6. A transferring sheet B is moved between the photo-sensitive member 1 and the transferring charger 6.
In FIG. 1, reference numeral 7 designates an original placed on a contact glass plate 8, and reference numeral 9 designates an exposure lamp. Light reflected by the original 7 is applied through a first mirror 10, a second mirror 11, an in-mirror lens 12, a fourth mirror 13 and a slit 14 to the photo-sensitive member 1.
One cycle of copying operation consists of the step of charging by the charging charger 3, the step of exposure by light applied through the slit 14, the step of development, the step of transferring by the transferring charger 6, and the step of fixing.
The effect that a bias voltage is applied to the electrode 5 by the bias device 4, will be described.
The charges left in the photo-sensitive member 1 in the preceding copying operation are removed by the discharging charger 2, and thereafter new charges are created in the photo-sensitive member 1 by means of the charging charger 3. Among these charges, the charges corresponding to the character region are left as they are but the charges corresponding to the white background region are removed during the exposure operation in the exposure section. Thus, the character region has a high potential, while the white background region has a low potential, thus forming a latent image on the photo-sensitive member 1.
In the next process, i.e., in the step of development, a toner is applied to the latent image. The toner is stuck to the character region, as a result of which the latent image is developed. However, in this operation, the toner is stuck to the white background region to an extent; that is, a portion which should not be developed is also developed.
In order to eliminate this difficulty, during the development a voltage having the same polarity as that of the latent image is applied, as a bias voltage, to the electrode 5, to attract the toner stuck to the white background region, thereby to prevent the adhesion of the toner to the white background region. In this case, the amount of toner stuck to the character region is somewhat reduced, and therefore it is necessary to minimize this effect by suitably selecting the bias voltage.
The developed image thus obtained is transferred onto a transferring sheet in the transferring section provided with the transferring charger 6, and is then converted into a copy image in the step of fixing.
As is apparent from the above description, the bias voltage applied to the electrode 5 should have a suitable value in correspondence to the character and white background regions of an original in any copying process. In general, a voltage charged in the photo-sensitive member 1 by the charging charger 3 is of the order of 1,000 volts, and the voltages of the portions, corresponding to the character and white background regions, of the member 1 exposed are of the order of several hundreds of volts and several tens of volts, respectively. Accordingly, it is suitable that the bias voltage is one hundred and several tens of volts (slightly higher than the potential of the white background region) to several hundreds of volts (slightly lower than the potential of the character region).
However, there is still a problem. That is, in the case where the developed image is transferred onto a number of sheets, the residual potential of the photo-sensitive member 1 after the transferring cannot be completely eliminated even with the discharging charger 2, and the residual potential V is increased with the number of times (n) of repetition as indicated in FIG. 2. (In FIG. 2, reference character V.sub.0 designates a residual potential due to the fatigue of the photo-sensitive member. After the copying operation, the residual potential is restored to the value V.sub.0.). Sometimes, the residual potential reaches several hundreds of volts. In this case, it is necessary to apply a voltage higher than the residual potential as the bias voltage to obtain a satisfactory copy image, because otherwise the portion having the residual potential of the previous copying operation would be developed. Accordingly, the bias voltage applied should be higher than the residual potential.
From the standpoint of the above description, the conventional bias device is unsatisfactory. The conventional bias devices are semi-automatic bias devices or automatic bias devices, none of which are satisfactory due to the following reasons: In the semi-automatic bias device, a resistor 19, a diode 20, a lower limit voltage V.sub.1 setting Zener diode 17 and an upper limit voltage V.sub.2 setting Zener diode 18 are connected to an electric source 16, as shown in FIG. 3, so that when the voltage induced by the photo-sensitive member is lower than the lower limit voltage V.sub.1, the lower limit voltage V.sub.1 is applied to the electrode 5, and when the voltage induced by the photo-sensitive member is higher than the lower limit voltage V.sub.1 and lower than the upper limit voltage V.sub.2, the potential of the electrode 5 is used as it is, and when the voltage induced is higher than the upper limit voltage V.sub.2, the upper limit voltage V.sub.2 is used as the bias voltage.
When the induced voltage is between the lower limit voltage V.sub.1 and the upper limit voltage V.sub.2, the induced voltage itself is used as the bias voltage, and therefore it is not the suitable voltage which is slightly higher than the potential of the white background region and the effect of keeping the background clear is low. This difficulty may be overcome by increasing the lower limit voltage V.sub.1. However, in this case, even in the case when the induced voltage is low, the bias voltage is high, and the character region other than the white background region is excessively biased, as a result of which the density is decreased.
On the other hand, in the automatic bias device, as shown in FIG. 4, a sensor electrode 21 is provided before an electrode 5 to which a bias voltage is applied, so that the potential of the photo-sensitive member corresponding to several tens of millimeters from the end of an original (the range in which usually only "a white image" is available, but no character image is available) is detected by the sensor electrode 21, and a voltage on which a voltage Va at the detected potential is superposed by an amplifier 22 is applied through a contact means 23 to the electrode 5. In FIG. 4, a circuit consisting of a transformer 24, diodes 25 and 26, and capacitors 27 and 28 is to apply a negative bias voltage through the contact means 23 to the electrode 5 to clean the latter 5.
In the automatic bias device shown in FIG. 4, the potential of the predetermined range from the end of the original is detected, and the bias voltage is determined from the potential thus detected. Therefore, no matter what ratio of the character region to the white background region may be in the rear end of the original, the bias voltage defined by the potential of the front end of the original is applied to the electrod 5, and the voltage superposed cannot be changed in that copying cycle. Therefore, this voltage is not always a bias voltage suitable for the rear half of the original. Thus, in the rear half of the original, the white background region may be developed, or the density of the character region may be decreased as the case may be.
As is apparent from the above description, any conventional bias device suffers from a disadvantage that contrast between the white background region and the character region is unstable.